1. Announcements Tutorial next Thursday, Oct 9 –Submit questions to me Mid-term schedule Go vote!

2. Today –Action Potential Conduction Next Lecture –Synaptic Transmission

3. Action Potential Conduction Axon hillock Region of neuron where AP usually starts

4. Action Potential Conduction Why are Action Potentials needed? 1.First look at current flow without APs. 2.Second look at current with APs

5. Passive Current Flow Record voltage Inject current axon

6. voltage distance 0.63V 0 0 Length constant V= Vo – 0.63Vo Inject current Record voltage

7. Passive Current 1.Current decays very rapidly along the length of an axon 2.The length constant is the distance over which the potential drops to 63% of the highest value 3.Typical length constants range only from 1-5 mm

8. Length Constant Depends on: 1.Resistance across the membrane (‘leakiness’) 2.Longitudinal resistance to current flow (varies with axon diameter)

9. Passive Current Flow Inject current axon Membrane Resistance Longitudinal Resistance

10. Action Potential Conduction Record voltage Stimulate Action Potential axon

11. Action Potential Conduction 1.APs constant amplitude at all points along the axon

12. Na+ Inject current

13. Na+ Inject current

14. Sequence of Events leading to AP propagation 1.Stimulus opens Na+ channels & cause AP 2.Depolarizing current flows down the axon 3.Local depolarization opens Na+ channels downstream & initiate a new AP 4.Na+ channels close (inactivate) & K+ channels open 5.Local depolarization opens Na+ channels downstream and initiate a new AP

15. Na+ Inject current

16. Conduction Velocity Record voltage Inject current axon Measure distance between recording sites Measure time between APs

17. 1.Axon diameter 2.Myelination Small unmyelinated  0.5 m/s Large myelinated  120 m/s

18. Myelinated nerve Myelin Formed by: Schwann cells (periphery) Oligodendrocytes (central) Node of Ranvier

19. Myelin

20. Na+ Saltatory conduction

21. Myelin Myelin increases speed of conduction by: 1.Increasing membrane resistance Reduces ‘leakiness’   length constant 2.Voltage-gated channels only at Node of Ranvier APs generated only at the Node

22. Mulitple Sclerosis Demyelination of axons –Impaired AP conduction –Symptom depends on nerves affected Optic nerve  blindness Motor nerves  weakness or paralysis

23. Summary & Key Concepts 1.Currents flow passively down axon decay described by length constant 2.Action potentials propagation due to sequential opening of Na+ channels in response to local depolarization 3.Conduction velocity determined by axon diameter and myelination - length constant 4.Myelin  trans-membrane resistance and Na+ channels only at Nodes Saltatory conduction

24. Sample Question AP conduction velocity will increase following an increase in the electrical resistance of the: A.Axon membrane B.Node of Ranvier C.Schwann cell